California Code, Water Code - WAT § 6003

(a) Any barrier that is or will be not in excess of six feet in height, regardless of storage capacity, or that has or will have a storage capacity not in excess of 15 acre-feet, regardless of height, shall not be considered a dam.

(c) For purposes of this section, a “weir” is defined as an agricultural water delivery structure with either mechanically or manually removable flashboards or gates that serves to regulate the flow of water in a stream and that functions as part of a federal flood control system as defined by Part 208 (commencing with Section 208.10) of Chapter II of Title 33 of the Code of Federal Regulations.